The present invention relates generally to apparatus and methods for heating and cooling polymer structures and, more particularly, relates to an apparatus and method for high-speed heating and cooling of a polymer structure.
During the heat treatment of a polymer structure, which includes welding of two polymer sheets, it is necessary to disengage a metal surface of a heating source from the polymer surface. The polymer surface tends to adhere to the metal heating surface at high temperatures and does not release from the metal surface until the temperature is lowered to well below the melting temperature of the polymer. Most traditional heat sealing technology does not include a forced cooling cycle to expedite the cooling process; however, a couple heat sealing techniques with forced cooling cycles are proposed in U.S. Pat. No. 4,075,818 to Wright et al. and U.S. Pat. No. 4,856,260 to Woo et al. Neither of these proposed techniques is adequate because the cooling cycle time is far too long for commodity production and the cooling process does not evenly cool the heated metal surface from one end to another.
Impulse heating of a metal tube with electric current can be regulated to achieve high temperatures at a relatively high speed (e.g., 100 milliseconds) because heat is generated within the wall of the tube. To cool the metal tube, the generated heat is removed from the tube wall starting from either the inner surface of the tube, the outer surface of the tube, or both the inner and outer surfaces of the tube. A common technique for removing heat starting from the inner surface of the tube is to convey a cooling medium such as air or fluid through the tube. A common technique for removing heat from a portion (less than half) of the outer surface of the tube is to contact the tube""s outer surface against a cool solid object. Heat can also be removed from the outer surface of the tube by immersing the tube in a cooled fluid. Heretofore, the overall cycle time has made it unfeasible to use impulse heating and forced cooling of the metal tube in commodity production. Also, if the metal tube is cooled by passing a cooling medium through it, the cooling medium cools the tube as it flows down the length of the tube but the cooling medium increases in temperature and becomes less effective at cooling the far end of the tube.
Accordingly, an object of the present invention is to provide an apparatus and method for high-speed heating and cooling of a polymer structure.
Another object of the present invention is to provide such an apparatus and method that minimizes temperature variations along the length of a tube structure employed in the apparatus.
These and other objects are realized by providing an apparatus for heating and cooling a polymer structure. The apparatus includes a tube structure having a conductive tube and a conductive strip. The tube is composed of a first conductive material having a first electrical resistivity, and the strip is composed of a second conductive material having a second electrical resistivity less than the first resistivity. The strip is connected to only a portion of an outer surface of the tube and extends along a length of the tube. The first and second conductive materials are selected such that passing an impulse of electric current through the tube structure induces a greater temperature increase in the strip than in the tube. To cool the heated strip, a cooling medium such as air or fluid is preferably passed through the metal tube.
A method of heating and cooling a polymer structure utilizes the above-described apparatus. In the method, an impulse of electric current is passed through the tube structure until the strip is heated to a predetermined sealing temperature; the strip and the polymer structure are brought into contact with each other for a dwell time sufficient to generate a heat seal in the polymer structure; and a cooling medium is passed through the tube to cool the strip to a predetermined release temperature that facilitates release of the strip from the polymer structure.
The above summary of the present invention is not intended to represent each embodiment, or every aspect of the present invention. This is the purpose of the figures and detailed description which follow.